Cassini Prepares for Last Epic Flyby of Saturn's Ocean Moon Enceladus

By Paul Scott Anderson, on December 17th, 2015

Cassini’s final close flyby of Enceladus will be on Dec. 19, 2015. Image Credit: NASA/JPL-Caltech

The Cassini spacecraft’s discoveries about the tiny moon Enceladus have been some of the most exciting of the entire mission at Saturn. What was once thought to likely be little more than a frozen ice world has turned out to be one of the best places in the Solar System to search for evidence of possible life, with its subsurface salty ocean and huge geysers of water vapor. Now, Cassini is preparing for its last close flyby of this intriguing moon and has also made new findings regarding the potential habitability of the ocean below as well as the nature of the geysers.

For its final close flyby (E22), Cassini will pass Enceladus at a distance of 3,106 miles (4,999 kilometers) on Saturday, Dec. 19, at 9:49 a.m. PST (12:49 p.m. EST). This will be the 22nd flyby of Enceladus; after that, Cassini will continue to observe Enceladus, but at much greater distances. Previous flybys through the plumes themselves, first seen in 2005, revealed that they contain water vapor, ice particles, salts, methane and organics. This flyby will focus on determining how much heat is emanating from the moon’s interior, which will help scientists better understand the environment in the ocean below the ice crust. Cassini’s Composite Infrared Spectrometer (CIRS) instrument will be used to observe heat flow across Enceladus’ south polar terrain. So far, data from Cassini has indicated that there is current geological activity on the seafloor including hydrothermal. It is thought that the ocean floor on Enceladus has hydrothermal vents just like on the ocean floors of Earth. Once thought to be localized to the south polar region, the moon’s subsurface ocean is now known to be global, just like on Europa.

Artist’s conception of the interior of Enceladus, depicting the global subsurface ocean, water vapor plumes and hydrothermal activity on the ocean floor. Image Credit: NASA/JPL-Caltech

Enceladus’ geological activity has been a delightful surprise for planetary scientists, since the moon is so small, only 330 miles (530 kilometers) in diameter, and so far from the Sun.

“We’ve been following a trail of clues on Enceladus for 10 years now,” said Bonnie Buratti, a Cassini science team member and icy moons expert at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “The amount of activity on and beneath this moon’s surface has been a huge surprise to us. We’re still trying to figure out what its history has been, and how it came to be this way.”

Right now, the south polar region is in darkness doing the long Saturnian winter, which makes it easier for Cassini to measure the amount of warmth coming from Enceladus itself and not just the Sun.

“Understanding how much warmth Enceladus has in its heart provides insight into its remarkable geologic activity, and that makes this last close flyby a fantastic scientific opportunity,” said Linda Spilker, Cassini project scientist at NASA’s Jet Propulsion Laboratory, Pasadena, California.

“The distance of this flyby is in the sweet spot for us to map the heat coming from within Enceladus – not too close, and not too far away. It allows us to map a good portion of the intriguing south polar region at good resolution,” said Mike Flasar, CIRS team lead at NASA’s Goddard Space Flight Center, Greenbelt, Maryland.

The geysers, over 100 known so far, erupt from the south polar region of Enceladus, from large fissures called Tiger Stripes. Water from the ocean below makes its way to the surface through the cracks, and then jets out into space in the form of vapor and ice particles. It’s something like an alien version of Yellowstone. Cassini has directly sampled some of that vapor, the analysis of which indicated that conditions below the surface were surprisingly hospitable for life, at least microscopic. Whether there actually is anything living down there is still unknown however. Cassini wasn’t designed to find direct evidence of life, but future missions being planned will do just that.

The last flyby, on Oct. 28 (E20), passed through the plumes again, at only about 30 miles (49 kilometers) above the moon’s surface. Data from the flyby is still being analyzed, and researchers are trying to determine how much hydrogen is present, which would result from water reacting with certain iron-rich types of rock in a process called serpentization. Similar processes have been found on Earth, such as in Lost City, a field of alkaline hydrothermal vents in the mid-Atlantic ocean. Hydrogen is a great source of energy for various forms of life. Calculating how much hydrogen there is in the Enceladean ocean has not been easy, due to complications with how the instruments involved take measurements. Hydrogen would be another strong piece of evidence for hydrothermal activity on the ocean floor.

“Hydrogen… has the potential to drive the synthesis of organic molecules – and much more,” said geochemist Christopher Glein with the University of Toronto and the Southwest Research Institute in Boulder, Colorado.

As also noted by Hunter Waite, INMS team lead at Southwest Research Institute in San Antonio, “Confirmation of molecular hydrogen in the plume would be an independent line of evidence that hydrothermal activity is taking place in the Enceladus ocean, on the seafloor. The amount of hydrogen would reveal how much hydrothermal activity is going on.”

Early results from the Oct. 28 flyby have added to the evidence for a high pH level in the water, about 11 or 12.

“This is remarkably high pH solution,” said Glein. “How did it get that way? We think that what happened on Enceladus, and which could still be happening today, is that there were geochemical reactions between magnesium and iron-rich rocks in Enceladus’ core reacting with ocean water. Those reactions led to the high pH,” he said.

“This really is a world with a habitable environment in its interior,” according to planetary scientist Jonathan Lunine, at Cornell University, who just presented the newest findings at the American Geophysical Union conference in San Francisco.

Map of the bluish Tiger Stripe fractures at the south pole, with geyser locations marked by white circles. Cassini’s flight path during the previous close flyby is marked by the red line. Image Credit: NASA/JPL-Caltech/Space Science Institute

The closest flyby ever was on Oct. 9, 2008, at an altitude of only 16 miles (25 kilometers).

There is also another curious finding about Enceladus’ plumes which scientists are trying to figure out. According to Space.com, Cassini has found that the level of activity has dropped by 30-50 percent since they were first discovered in 2005. Those results were also presented at the American Geophysical Union meeting, on Dec. 14, by lead author Andrew Ingersoll, of the California Institute of Technology in Pasadena. He stated that the finding does not mean the geysers are shutting down, but the fissures may be narrowing from the accumulation of material.

“But why they would all act together is totally beyond me,” he said.

It may also simply be that the water pressure varies over time. In 2013, it was reported that the strength of the geysers depended on where Enceladus is in its orbit around Saturn. The moon’s orbit is elliptical, and the most material appeared to erupt from the fissures when the moon was farthest away from Saturn. The reasoning was that the cracks are larger at those times, allowing more material to escape into space. The new studies indicate that this dimming of activity has lasted throughout 2015.

One of the most well-known and beautiful images of Enceladus, showing the geysers erupting from the fractures at the south pole. Image Credit: NASA/JPL-Caltech

As for the search for life, there are currently two proposed follow-up missions competing to do that. The Enceladus Life Finder (ELF) would fly through the plumes and analyze them, like Cassini, but with more advanced instruments. The MASPEX instrument would analyze gas coming from the vents, while the Enceladus Icy Jet Analyzer would focus on solid particles in the plumes. ELF would be capable of finding and identifying amino acids, the building blocks of life.

“The plume of Enceladus is waiting for us. It could be the place where we find out if life had a second genesis in our own Solar System,” Lunine said in a talk at the American Geophysical Union meeting. “We must go back.”

The LIFE (Life Investigation For Enceladus) mission however, would collect samples and then bring them back to Earth. Peter Tsou, head of the proposed mission, says it is more likely to produce solid results than ELF:

“We don’t even have a set of instruments on Earth to detect life. We aren’t even sure what life is. So if we go out there with any in situ instruments, they will always be extremely limited and the information you get won’t be very definitive. A more productive means to learn of the possibility of life would be to bring samples to terrestrial laboratories for the scientists of the world to reach a consensus.”

LIFE wouldn’t have as many flybys however, and there is always the concern about possible contamination, if any kind of organisms we’re actually present in the samples.

“They [NASA] want to make sure we do not bring any live germs to kill everybody on Earth,” he said. As Lunine noted, “We have a much lower-risk approach to understanding whether there’s life on Enceladus.”

“Cassini’s legacy of discoveries in the Saturn system is profound,” said Spilker. “We won’t get this close to Enceladus again with Cassini, but our travels have opened a path to the exploration of this and other ocean worlds. We’ll continue observing Enceladus and its remarkable activity for the remainder of our precious time at Saturn. But these three encounters will be our last chance to see this fascinating world up close for many years to come.”

The coming flyby this Saturday will be another great opportunity for scientists to learn more about Enceladus and its potential to support life. Enceladus is also an excellent example of the old saying that truth is often stranger than fiction. Indeed.

An online toolkit for all three of the last three flybys is available here.

Continue to follow AmericaSpace for more updates as they become available. Further images and science results will be released in the coming days and weeks, so stay tuned. More information about the Cassini mission is also available here.